what work needs to be done in the raw converter to be able to correctly deal with the less well defined saturation level?

Linearization, normally, based on photon transfer curve. Pretty heavy equipment needed if doing it precisely for that small highlight region; but in practice just three points, the start, the saturation, and some midpoint, using a spline. The other thing necessary is to deal with pattern noise, also not a huge problem.

If the curve near actual FWC is easily characterized, why doesn't everybody do it?

It is per camera characterization.

I see, so not very practical for mass production houses like Canon and Nikon I guess.

And, sorry, Jack, but "everybody" type questions are one of the sort that make little sense to me